Piston pump



FIG. 1 is a perspective view of a piston pump showing, my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevation thereof;

FIG. 4 is a right end view thereof;

FIG. 5 is a left end view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a rear elevation view thereof.

The ornamental design for “piston pump,” as shown and described.